1. Field of the Invention
The present invention relates to a sheet folding apparatus that performs folding processing on a sheet, an image forming apparatus equipped with the sheet folding apparatus, and an image forming system.
2. Description of the Related Art
A conventional sheet folding apparatus is known that performs folding processing in which a curved portion (bend) formed on a sheet is nipped and conveyed between a pair of folding rollers that is a pair of roller members to form a folded part.
A sheet folding apparatus described in Japanese Laid-open Patent Publication No. 2007-277006 includes a pair of upstream conveying rollers and a pair of downstream conveying rollers that are disposed along a sheet conveying direction and that hold a part of a sheet to apply conveying force to the sheet. The sheet folding apparatus also includes a pair of folding rollers that forms a folded part on the sheet by nipping a curved portion formed by bending the sheet between the pair of upstream conveying rollers and the pair of downstream conveying rollers. The following describes how the sheet folding processing is performed. The pair of upstream conveying rollers and the pair of downstream conveying rollers each hold a part of the sheet, and the pair of downstream conveying rollers applies conveying force to the sheet to reverse the sheet upstream in the sheet conveying direction, thereby forming a curved portion on the sheet between the pair of upstream rollers and the pair of downstream rollers. The curved portion thus formed is then guided to and nipped between the pair of folding rollers to form a folded part on the sheet.
It is desirable that a sheet folding apparatus can perform various types of folding processing such as twofold processing, z-shaped folding processing, outer-threefold processing, and inner-threefold processing. Such various types of folding processing can be performed by changing the position of a folded part on a sheet in a sheet conveying direction depending on the types of the folding processing. The position of the sheet folded part in the sheet conveying direction can be changed by changing a protrusion amount of the leading end of the sheet from the pair of downstream conveying rollers when the sheet is stopped depending on the types of the folding processing.
It is conceivable that the sheet folding apparatus includes a sheet leading end detection sensor for stopping the sheet with the leading end of the sheet protruding from the pair of downstream conveying rollers by a predetermined protrusion amount by controlling the pair of upstream conveying rollers and the pair of downstream conveying rollers on the basis of a detection timing of the leading end of the sheet detected by the sensor. For example, the sheet leading end detection sensor is disposed upstream of the pair of upstream conveying rollers in the sheet conveying direction, and after a predetermined time has passed since the sheet leading end detection sensor detected the leading end of the sheet, the sheet folding apparatus stops the pair of upstream conveying rollers and the pair of downstream conveying rollers to stop conveying the sheet.
The sheet is held only by the pair of upstream conveying rollers during travelling from the pair of upstream conveying rollers toward the pair of downstream conveying rollers. Because of this, the behavior of the sheet between the pairs of upstream and downstream conveying rollers varies depending on, for example, stiffness or curling of the sheet. When the behavior of the sheet between the pairs of upstream and downstream conveying rollers varies, the sheet takes different courses in a sheet conveying path from the pair of upstream conveying rollers to the pair of downstream conveying rollers, thereby changing a timing at which the leading end of the sheet reaches the pair of downstream conveying rollers. Thus, when the sheet folding apparatus stops conveying the sheet after a predetermined time has passed since the sheet leading end detection sensor detected the leading end of the sheet, the protrusion amount varies. Consequently, the sheet folding apparatus cannot stop the sheet at an exact position, thus, positions of the sheet folded part varies in the sheet conveying direction, thereby being unable to perform good folding processing.
In view of the above, there is a need to provide a sheet folding apparatus that can reduce variations of the position of a folded part formed on a sheet in a sheet conveying direction, and an image forming apparatus and an image forming system equipped with the sheet folding apparatus.